President George W Bush has praised John McCain's service and leadership in a speech to the Republican convention.
Speaking via video-link from the White House, he told delegates in St Paul, Minnesota, that Senator McCain was "a great American and the next president".
Mr McCain is due to be nominated on Thursday as the party's presidential candidate for the 4 November election.
Senator Joe Lieberman, the Democratic vice-presidential nominee in 2000, said Mr McCain was the man to unite the US.
In his eight-minute address, Mr Bush described Mr McCain as a president ready to make the tough decisions needed "in a dangerous world".
John McCain's life is a story of service above self
President George W Bush
Speech in full: George W Bush
Wife's role in the show
Republican Convention diary
Convention in pictures
"John McCain's life has prepared him to make those choices. He is ready to lead this nation," Mr Bush said.
He also spoke of Mr McCain's life as "a story of service above self" and emphasised the "independence and character" he showed in backing the administration's "surge" strategy of pouring more forces into Iraq.
President Bush cancelled his planned opening night speech amid concerns that overt political campaigning would play badly with voters when the US Gulf Coast was suffering the effects of Hurricane Gustav.
But many Republicans would have been glad he was not there in person, as the convention aims to define Mr McCain as very different to his unpopular predecessor, says the BBC's Adam Brookes in St Paul.
Bipartisan politics
Giving the night's keynote speech, Mr Lieberman spoke of Mr McCain as "the best choice to bring our country together and lead our country forward".
Mr Lieberman - a former Democrat who was Al Gore's running mate in 2000 - told those who wondered what he was doing at a Republican convention: "I'm here to support John McCain because country matters more than party."
Senator Lieberman on why he backs John McCain
He praised Mr McCain for his independence, commitment to bipartisan politics and determination to root out corruption and fight special interests in Washington.
"Only one leader has shown the courage and the capability to rise above the smallness of our politics and get big things done for our country and our people," Mr Lieberman said.
Mr Lieberman described the Democratic presidential candidate, Barack Obama, as a "gifted and eloquent young man", but added: "Eloquence is no substitute for a record, not in these tough times."
Former Senator Fred Thompson, who ran against Mr McCain in the party's primaries, delivered fierce criticism of the Democrats.
Mr Obama, he said, was "the most liberal, most inexperienced nominee to ever run for president".
Mr Thompson praised Mr McCain's military service, his courage while a prisoner of war in Vietnam and his commitment to reform in Washington.
He also rounded on the media for their scrutiny of Mr McCain's vice presidential choice, Sarah Palin, and her family.